Several references are listed at the end of the disclosure portions of this patent specification and are referred to below by numbers in parenthesis. These references as well as prior patents identified in this patent specification are hereby incorporated by reference.
In the U.S. breast cancer mortality is second only to that of lung cancer for women. Because of its role in early tumor detection, x-ray mammography has become the most commonly used tool for breast cancer screening, diagnosis and evaluation in the United States. A mammogram is an x-ray image of inner breast tissue that is used to visualize normal and abnormal structures within the breasts. Mammograms provide early cancer detection because they can often show breast lumps and/or calcifications before they are manually palpable.
While screening x-ray mammography is recognized as the most effective method for early detection of breast cancer, it also presents challenges in that in some cases it may be difficult to determine whether a detected abnormality is associated with a cancerous or benign lesion. One reason for this is that a mammogram Mp is a two dimensional projection image representating a three dimensional structure, and overlapping structures in the compressed breast may confound image interpretation and diagnosis. A second reason is that the x-rays that are often used to obtain the images have energies that are in a range that helps achieve a desirable Signal to Noise Ratio (SNR) but at the same time may cause the x-rays to be attenuated to a similar degree by breast structures that may have different clinical significance.
Efforts to improve the sensitivity and specificity of breast x-rays have included the development of breast tomosynthesis systems. Breast tomosynthesis is a three-dimensional imaging technology that involves acquiring images of a stationary compressed breast at multiple angles during a short scan. The individual projection tomosynthesis images Tp taken at respective angles of the imaging x-ray beam relative to the breast are then computer-processed into a series of reconstructed tomosinthesis slice images Tr each representing a respective slice of the breast. The Tp and/or Tr images can be displayed individually or concurrently or in a dynamic ciné mode. Breast tomosynthesis mammography [see references 14-19] typically uses a field digital mammography (FFDM) platform. In one example, an x-ray tube moves in an arc above the breast and a series of 11 to 22 low dose x-ray 2-D tomosynthesis projection images Tp is obtained. The sum of the dose from all of the 2-D tomosynthesis projection images Tp is similar to the dose from a single conventional digital mammogram Mp. These low-dose 2-D tomosynthesis projection images Tp are reconstructed into a series of 3-D slice images Tr each representing a slice of the breast where each slice is, for example, 1-5 mm thick. The slice images typically conform to planes parallel to the platform supporting the breast during image acquisition, but could be oriented differently. An advantage of breast tomosynthesis compared to conventional mammography is that by showing the breast as a series of slices rather than a single mammogram, a lesion may be seen with greater clarity because much of the superimposed tissue present in a conventional mammogram has been removed.
Reconstructed tomosynthesis slice images Tr reduce or eliminate problems caused by tissue overlap and structure noise in two-dimensional mammography imaging. Digital breast tomosynthesis also offers the possibility of reduced breast compression, improved diagnostic and screening accuracy, fewer recalls, and 3D lesion localization. An example of a multi-mode breast tomosynthesis/mammography system is described in commonly assigned U.S. Pat. No. 7,869,563. Other aspects of breast tomosynthesis and mammography are described in commonly assigned U.S. Pat. Nos. 7,991,106, 7,760,924, 7,702,142, and 7,245,694, which are hereby incorporated by reference.
In an effort to address challenges in differentiating breast cancer from benign abnormalities in breast x-ray imaging, consideration has been given to contrast-enhanced and dual-energy imaging. In contrast-enhanced imaging, a contrast agent that may be iodine-based is introduced into the breast, typically through an injection in a vein remote from the breast, and x-ray images are taken after (as well as possibly before) the contrast agent has reached the breast. The contrast agent helps highlight vascularity in the breast. If images of the same breast taken before and after the arrival of the contrast agent in the breast are subtracted from each other (and absent breast motion between the times the two images are taken), breast vascularity may be appear even more clearly in the resulting subtraction image. This may assist in differentiating cancer from benign tissue because it is believed that breast cancers release angiogenesis factors that increase the formation of small vessels near the tumor (1, 2). (The Arabic numbers in parenthesis refer to respective publications listed at the end of this patent specification.) It is believed that the growth of breast cancer is dependent on angiogenesis, and that these vessels differ from normal vessels in that they have increased permeability and are often tortuous. Imaging of the vessels around a tumor is believed to allow improved detection of breast cancer.
MRI (Magnetic Resonance Imaging) can be used with contrast enhancement to help characterize breast cancers by imaging the vascular network near a breast cancer (3). Although contrast enhanced breast MRI (CEMRI) can be effective in imaging breast cancer it has limitations including high cost, long procedure time, enhancement of benign abnormalities such as fibroadenomas, and inability to image women with metal clips or claustrophobia. Typically, the contrast agent used in CEMRI is gadolinium-based and is different from the contrast agents used in x-ray imaging.
X-ray imaging also can use contrast enhancement to improve cancer detection. The use of contrast agents such as iodine with x-ray methods has been suggested for imaging the vascular network near a breast cancer. These x-ray imaging methods include breast CT (4, 5), breast tomosynthesis (6, 7) and digital mammography (8-13). Contrast enhanced x-ray mammography (CEM) may improve the conspicuity of breast cancers (8-13). It has also been suggested that CEM may provide improved specificity compared to CEMRI because fewer benign lesions enhance (13). These studies are small and may need to be validated with larger trials.
In x-ray mammography, contrast enhanced mammography has been evaluated using two methods. The first involves subtraction of images obtained pre- and post-contrast (9). This method is referred to as time subtraction. The second method is referred to as dual-energy contrast imaging. In this method images are obtained at low energy and high energy after the injection of contrast. The images are obtained at energies above and below the k-edge of iodine (33.2 keV) when iodine-based contrast agent is used. At x-ray energies just above the k-edge the absorption of x-rays is increased resulting in an increase of contrast from the iodine contrast agent in the high energy image. Subtraction of these two images enhances iodine contrast while suppressing the contrast of normal breast anatomy. An advantage of dual-energy contrast imaging mammography is that both images may be obtained in a very short time and therefore the images may be subtracted with little patient motion. This is not true for subtraction of pre- and post-contrast images since typically there will be more than a minute separating the acquisition of the two images.
One goal of any x-ray imaging system is to obtain the highest quality images to reduce the occurrence of false positive and false negative diagnoses. It would be desirable to identify a system and method for acquiring x-ray images to alleviate issues associated with specificity and sensitivity in current designs.